Problem 95 - Objective Biology for NEET

Question

Which of the following is correct about parapodia? (a) They are longitudinal muscles which help in locomotion. (b) They are circular muscles which help in locomotion. (c) It helps in swimming. (d) It is the dorsal appendages of nereis.

Step-by-Step Solution

Answer

The correct answer is (c) It helps in swimming.

Step 1: Understanding Parapodia

Parapodia are the small, fleshy, paired appendages that extend from each segment of polychaete annelids (a class of worms). They have multiple functions in the day-to-day functions of these creatures.

Step 2: Analyze Each Option

Next step is to look at each option individually. \n(a) Parapodia are not muscles; they are appendages made up of muscular tissue. So, this statement is not correct. \n(b) As for choice B, parapodia are not circular muscles. This statement, therefore, is also incorrect.\n(c) Parapodia do indeed aid in swimming. They are powerful and undulate in a wave-like motion, propelling the worm through water. Therefore, this answer is plausible. \n(d) Parapodia are the appendages of nereis, not just the dorsal ones. They are present on many segments throughout the worm's body. So, this statement is partly correct, but may be misleading as it implies parapodia are only dorsal in location.

Step 3: Select the Correct Answer

Among the given options, only option (c) describes an accurate and complete function of parapodia. So, (c) It helps in swimming, can be concluded as the correct answer.

Key Concepts

Polychaete Annelids

Polychaete annelids are a fascinating and diverse group of segmented worms that make up the class Polychaeta. One distinguishing feature of these creatures is the presence of parapodia, which serve as locomotive apparatuses. These parapodia are bristling with setae, or little bristles, that aid in their movement.

They inhabit various environments, ranging from the depths of the ocean to the intertidal zones. These annelids exhibit great adaptability and can be either free-moving or sedentary. The free-moving polychaetes, such as the ragworm or the clamworm, are often agile and quick, utilizing their parapodia effectively for swimming.

Polychaetes also play a critical role in the benthic ecosystem as they participate in the digestion of organic materials, hence contributing to nutrient cycling. The presence of parapodia thus not only serves as a means of motility but also aids in their ecological functions.

Locomotion in Annelids

• Segmentation: Annelid bodies are segmented, which allows for flexibility and precise movements.
• Coordinated Muscle Action: The longitudinal and circular muscles of each segment work in opposition, allowing the annelid to elongate and contract, creating a thrashing motion that propels them forward.
• Setae: The tiny bristles found on most annelids give them grip in soil or sediment, facilitating movement.

For many annelids, locomotion is primarily terrestrially based, involving muscle contractions that allow them to burrow or crawl. Polychaete annelids, however, have evolved parapodia that are more specialized for swimming. These appendages not only aid in aquatic propulsion but can also help in crawling when these organisms find themselves on the substrate.

Annelid Anatomy

Annelid anatomy is both complex and beautifully adapted for survival in varied environments. Their bodies are segmented, providing both structural advantages and a means to compartmentalize organs. Each segment contains portions of the nervous and circulatory systems, such that damage to one part does not necessarily incapacitate the organism.

Circular and Longitudinal Muscles:

These muscles allow for the constricting and elongating movements essential for annelid locomotion. When the circular muscles contract, the segment becomes longer and thinner; conversely, when the longitudinal muscles contract, the segment becomes shorter and thicker.

Digestive System:

The digestive tract of an annelid is typically straightforward, running from the mouth to the anus, with a few organs in between for nutrient absorption and waste processing.

Nervous System:

The annelid's nervous system includes a simple brain and a nerve cord that runs the length of the body with ganglia in each segment, coordinating movement and sensory input.

Understanding the intricacies of annelid anatomy provides a window into how these organisms have successfully adapted to their environments over millions of years. Their musculature, including unique structures like parapodia in some species, reflects a fascinating evolutionary response to the challenges of their diverse lifestyles.